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501

Syntax Error in Parameters

The command parameters contain syntax errors, often due to malformed email addresses.

What does this error mean?

SMTP error 501 means the command was recognized but its parameters were invalid:

  • Malformed email address format
  • Invalid characters in addresses
  • Missing required parameters
  • Incorrect encoding or special characters

Common Causes

  • alternate_email
    Invalid Email Format

    The email address contains invalid characters or is improperly formatted.

  • text_fields
    Special Characters

    Unencoded special characters in the sender or recipient fields.

  • format_quote
    Encoding Issues

    Character encoding problems in email headers or addresses.

    • How to Fix

    1. Verify Email Addresses

    Check that all email addresses are properly formatted ([email protected]) without extra spaces or invalid characters.

    2. Check Special Characters

    Remove or properly encode special characters in display names and addresses.

    3. Use Standard Encoding

    Ensure your mail client uses standard UTF-8 encoding for international characters.

    Example response 
    501 5.1.3 Invalid address format
501 Syntax error in parameters or arguments
501 5.5.4 Invalid address

    Related SMTP Errors

    500 Syntax Error 553 Invalid Mailbox Name 550 Mailbox Not Found 504 Parameter Not Implemented

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